The present invention relates to a light beam scanning device in which scanning speed irregularities of a light spot in a scanning plane are corrected.
A variety of light beam scanning devices have been proposed in the art which operate to record data on a recording material placed on its light spot scanning plane or to read data from an original placed thereon. Most of the conventional devices employ a vibrating mirror such as a galvanometer or a rotary multi-surface mirror as a light beam deflector. For such devices, it is desirable that the light spot scan the scanning plane at a constant speed. If the scanning speed is irregular, a recorded pattern or a pattern obtained by reproducing read signals will also be irregular. For instance, in the case where a galvanometer is employed as the light beam deflector, a scanning speed irregularity attributed to a combination of inaccuracies in the waveform of sawtooth wave drive signals, inaccuracies of response of the galvanometer and inaccuracies in the repetition rate can produce a considerably great irregularity in a recorded image. This irregularity increases as the scanning speed increases. In the case also where a rotary multi-surface mirror is employed as the light beam deflector, it is considerably difficult to rotate the mirror at a constant speed, and accordingly the light spot scanning speed is liable to be irregular.
A method of substantially correcting light spot scanning speed irregularities described above has been known in the art. In accordance with that technique, a grid pattern having transparent and opaque portions arranged alternately with a predetermined period in the direction of scanning is placed in a plane which is substantially equivalent to a scanning plane for recording or reading information. A photoelectric signal obtained through the grid pattern is used as a reference signal in the scanning operation.
Examples of a light beam scanning device for practicing the conventional methods are shown in FIGS. 1 and 2. In FIGS. 1 and 2, reference numeral 1 designates an optical deflector, 2 a scanning lens, 3 a scanning plane (surface) on which a recording material or an original is placed, 5 a grid pattern, 6 a condenser lens, 7 an optical detector, 8 a first laser beam (indicated by a solid line) adapted to scan the recording material or the original, and 9 a second laser beam (indicated by a broken line) adapted to scan the grid pattern.
In the example shown in FIG. 1, the first laser beam 8 and the second laser beam 9 are applied along substantially the same axis to the optical deflector 1. After being deflected by the optical deflector 1, the laser beams are applied through the scanning lens 2 to a beam separating mirror 4 whereby they are separated so that the first laser beam scans the scanning plane 3 on which the recording material or the original is placed while the second laser beam scans the grid pattern 5. In this case, the length of the grid pattern 5 in the scanning direction is equal to or more than the length of the recording material or the original in the scanning direction. For instance, in the case of a facsimile device or a printer, the length must be at least 210 mm (equal to the length of the short side of an A4 size sheet). Accordingly, in order to collect the laser beam and to direct it onto the optical detector 7 after the beam has passed through the grid pattern 5 and has been modulated thereby, the condenser lens 6 must have a considerably large diameter or a large bundle of optical fibers must be used. Although a Fresnel lens of large diameter is readily available, its light collecting characteristic is not sufficiently great. A bundle of optical fibers is expensive.
In the case of FIG. 2, the first laser beam 8 and the second laser beam 9 applied to the optical detector 7 form an angle, i.e., the laser beams 8 and 9 are applied to the optical detector 7 at different angles. More specifically, after being deflected by the optical deflector 1, the first laser beam 8 passes through the first scanning lens 2 and scans the scanning plane 3 on which the recording material of the original is placed. On the other hand, the second laser beam 9 falling on the rear surface of the optical deflector 1 is deflected thereby. The second laser beam 9 thus deflected passes through the second scanning lens 2' and scans the grid pattern 5.
If, in this case, the focal distance of the second scanning lens 2' is made shorter than that of the first scanning lens 2, the length of the grid pattern 5 in the scanning direction can be shorter than the length of the recording material or the original in the scanning direction. Accordingly it is then possible to use an ordinary glass lens as the condenser lens 6. However, this optical system is still disadvantageous in that, in order for the optical system to perform properly, the image forming characteristics of two scanning lenses 2 and 2' should be the same. Thus, since it is desirable that the lenses be f.theta. lenses, the manufacturing cost of the optical system increases.
As is apparent from the above description, the conventional method of correcting the scanning speed irregularity of a light spot in a scanning plane suffers from drawbacks in that the performance is insufficient and the manufacturing cost of a device for practicing the method is high.
Accordingly, an object of the invention is to provide a light beam scanning device in which all of the above-described difficulties accompanying a conventional method have been eliminated and the scanning speed irregularities of a light spot in a scanning plane are corrected, which has a high performance, and low manufacturing cost.